Olefins, also called alkenes, are unsaturated hydrocarbons whose molecules contain one or more pairs of carbon atoms linked together by a double bond. When subjected to a polymerization process, olefins are converted to polyolefins, such as polyethylene and polypropylene. Ziegler-type polyolefin catalysts, their general methods of making, and subsequent use, are known in the polymerization art. While much is known about Ziegler-type catalysts, there is a constant search for improvements in their polymer yield, catalyst life, catalyst activity, amenability to use in large scale production processes, and in their ability to produce polyolefins having certain properties.
Conventional Ziegler-Natta catalysts comprise a transition metal compound generally represented by the formula:MR+x where M is a transition metal, R+ is a halogen or a hydrocarboxyl, and x is the valence of the transition metal. Typically, M is a group IVB metal such as titanium, chromium, or vanadium, and R+ is chlorine, bromine, or an alkoxy group. The transition metal compound is typically supported on an inert solid, e.g., magnesium chloride.
The properties of the polymerization catalyst may affect the properties of the polymer formed using the catalyst. For example, polymer morphology typically depends upon catalyst morphology. Acceptable polymer morphology differs for each class of production process (e.g., slurry loop, bimodal, gas phase, etc.), but typically includes uniformity of particle size and shape and an acceptable bulk density. Furthermore, there is a need in the art of preparing polymers to minimize the number of very small polymer particles (i.e., fines) to avoid plugging polymer transfer lines or solvent recovery systems.